New Phenomena And Corresponding Mechanisms Found In Pressurized Superconducting Alloys And Compounds

To begin with,the properties and related theories of both conventional and high-temperature cuprate superconductivities are introduced.Then,apparatus and methods of high-pressure experiments on superconductivity are illustrated.In the following,our works are presented,where transition-metal elements and alloys that corresponding to conventional superconductivity,Bi-based cuprate that corresponding to high-temperature superconductivity,and quasi-one dimensional superconductor K₂Mo₃As₃were investigated.Starting with the research on alloys,we found the superconducting transition temperature（_c）of high-entropy alloy（Sc Zr Nb Ta）_0.6（Rh Pd）_0.4 is unchanged by large volume shrinkage.Such robust superconductivity against large volume shrinkage is defined as RSAVS.Comparing with previously found RSAVS alloys or elements,we found that these superconductors are composed of transition-metal elements,and the body-centered cubic“lattices”of them sustain in RSAVS state.The pressurized electronic structures of them are obtained by first-principle calculations.It is found that in RSAVS state,although density states of orbitals d_xy,d_yz,and d_xz are dominant,density states of d²²_x-y and d_z² at Fermi level keep unchanged,which correspond to the behavior of_c.The amounted superfluid density of d²²_x-y and d_z² fits well to experimental values by directed comparing or by checking Homes’law,indicating they are related to RSAVS superconductivity.Next,the high-pressure researches on superconducting compounds are presented.Optimal-doped Bi₂Sr₂Ca Cu₂O_8+δis investigated by employing high-pressure and in-situ measurements on resistance,magnetoresistance,X-ray diffraction（XRD）,and susceptibility.A pressure-induced two-dimensional（2D）and BKT superconducting transition is found in the strange metal state above 2.8 GPa.The pressure dependence of_c of this 2D superconductivity is similar to that of bulk superconductivity.XRD shows that flat Cu O₂ plane probably benefits 2D superconductivity.Similar measurements on under-doped and over-doped Bi₂Sr₂Ca Cu₂O_8+δdemonstrate that pseudo gap is likely detrimental to 2D superconductivity and strong correlation may be essential to 2D superconductivity.These results are vital for understanding high-temperature superconductivities.Last,a high-pressure investigation of quasi-one dimensional compound K₂Mo₃As₃ is narrated,where a pressure-induced reemergence of superconductivity is characterized.The_c of K₂Mo₃As₃ firstly decreases with elevated pressure and disappears at 8.7 GPa,and remains undetectable until 18.2 GPa,where another superconductivity forms.The upper critical fields of this high-pressure superconducting phase at 0 K unexceed Pauli limit,which are deviated from the situation of low-pressure superconducting phase,indicating strong correlation is probably suppressed in high-pressure phase.High-pressure XRD shows the space group of K₂Mo₃As₃ remains unchanged up to 51.6 GPa,but the variation of lattice parameter c is uncommon.Moreover,the variation of c corresponds well with the variation of superconductivity,which shows a correlation between them.First-principle calculations show three orbitals,d_z²,p_x,and p_y of K₂Mo₃As₃ correlate to superconductivity or parameter c,indicating the importance of Mo As chain.This is the first experimental case that a pressure-induced reemergence of superconductivity is found in quasi-one dimensional materials,and the corresponding in superconductivity,lattice structure,and electronic structure is obtained.To conclude this dissertation,a summary is drawn,and the distinction between superconducting alloys and superconducting compounds is narrated.